1. Field of the Invention
The present invention relates to an apparatus for use in a display device and a method for fabricating the same, more particularly, to an apparatus for use in a display device in which gray scale is easily displayed, and a method for fabricating the same.
2. Discussion of Related Art
Generally, circuits with complementary metal oxide semiconductor (CMOS) thin film transistors (TFTs) are used for driving active matrix liquid crystal displays (AMLCD), active matrix organic electroluminescence display devices (AMOLED), and active matrix flat panel display devices, including image sensors.
In an active matrix flat panel display device, N-type metal oxide semiconductor (NMOS) TFTs, which are typically used in circuits and as switching transistors, and P-type metal oxide semiconductor (PMOS) TFTs, which are typically used as driving transistors, have different required characteristics.
Particularly in an AMOLED, TFTs used in circuits and as switching transistors must have a low threshold voltage and a low S-factor, which is the reciprocal of the slope of the curve of source/drain current according to a change of gate voltage, and driving TFTs must have a high S-factor for displaying gradation.
Korean Patent Laid-open Publication No. 1995-33618 discloses a method for fabricating a TFT by varying the thickness of the polysilicon film depending upon whether the TFT is used in a circuit or as part of a pixel part, thereby varying the TFT characteristics.
However, varying polysilicon film thickness per TFT positions is a complicated process, requiring control of many variables to lower the characteristics of driving transistors only.